This invention relates to a pneumatic radial tire. More particularly, the present invention is concerned with a radial tire for high speed travelling which is excellent in high speed durability.
Conventional radial tires for high speed travelling include one in which a belt layer comprised of a steel cord is provided on the tread portion and is covered in both edge portions or in the entirety thereof with a reinforcing cover made of a nylon cord or the like. The reinforcing cover suppresses the lift of the belt layer caused by the action of centrifugal force during high speed travelling, thereby preventing edge separation of the belt layer, which contributes to an improvement in high speed durability.
In recent years, in order to further enhance the effect of improving high speed durability attained by the provision of the above-mentioned reinforcing cover, a radial tire as shown in FIGS. 3 and 4 was proposed. Such a radial tire is characterized in that each of the both edge portions of the steel belt layer 2 is covered with a narrow reinforcing cover 3 and, at the same time, the whole of the belt layer is covered with a wide reinforcing cover 4. However, in this radial tire, the reinforcing covers 3 and 4 inevitably have splice portions 3s and 4s. The rigidity of the splice portions is higher than that of the other portions, which brings about a rigidity difference in the circumferential direction of the tire in the tread portion. Therefore, when a number of reinforcing covers are used as shown in FIGS. 3 and 4, there also arises a difference in the degree of rigidity difference in the circumferential direction of a tire between the shoulder portion and the central portion. More precisely the rigidity difference in the central portion is larger than that in the shoulder portion.
The occurrence of the rigidity difference in the circumferential direction of a tire in the tread portion lowers uniformity of the tire, which causes occurrence of vibration during high speed travelling. This not only spoils riding comfort but also frequently causes damages to the splice portions.
The reinforcing cover is provided to suppress the lift of the belt layer made of a steel cord caused by the action of centrifugal force. Therefore, in order to enhance the effect, it is necessary for the reinforcing cover to have a given rigidity. In general, the rigidity of the reinforcing cover can be enhanced by increasing the denier number (thickness) of the cord constituting the reinforcing cover. However, mere increase in the denier number causes an increase in the difference in level at the cut portion because of an inevitable increase in the thickness of the reinforcing cover. This further enlarges the rigidity difference between the splice portions and the other portions, thus leading to lowering in the uniformity of the tire. Moreover, this tends to bring about damage to the uneven portions during high speed travelling.